Magnetic resonance imaging (MRI) involves the transmission of radio frequency (RF) energy. RF energy may be transmitted by a coil. Resulting magnetic resonance (MR) signals may also be received by a coil. In early MRI, RF energy may have been transmitted from a single coil and resulting MR signals received by a single coil. Later, multiple receivers may have been used in parallel acquisition techniques. Using multiple receivers facilitates speeding up signal reception, which in turn may reduce scan time. Similarly, multiple transmitters may be used in parallel transmission techniques. Using multiple transmitters may facilitate speeding up a transmission process, which in turn may facilitate volumetric excitation, selective isolation, and other very high speed features. However, conventional parallel transmission techniques have encountered issues with scaling, fidelity, and synchronization.
Conventional systems may have attempted to parallelize their existing RF transmitters. Thus, conventional systems may have relied on multiple, individually powered, single channel, analog-in-analog-out RF transmitters for parallel transmission. These systems tended not to scale well due to cabling duplication, power transmitter duplication, control duplication, and other issues. Even when a small number (e.g., 4) of transmitters were employed, these systems may not have produced desired fidelity. For example, conventional systems may have had complicated power distribution management and may have been difficult to synchronize. Additionally, conventional systems typically had poor isolation between coils, resulting in degraded performance. Furthermore, these systems may have required each element in an array to be tuned and matched, which is a very time-consuming procedure.
Conventional systems may also have been limited by their use of relatively low power (e.g., <50 W), low efficiency class A or class AB amplifiers. While some systems may have included on-coil series and/or shunt-fed class-D amplifiers, even these conventional systems have suffered from several limitations including inadequate detuning and low efficiency. Proposed systems that indicate that they may achieve higher efficiency still appear to lack adequate control mechanisms. Due, at least in part, to these limitations, conventional systems may not have produced desired levels of amplitude and/or phase control and thus may have had less than desirable fidelity.
U.S. Pat. No. 7,671,595 (“the '595 patent) to Griswold et al. which issued on Mar. 2, 2010, and is entitled “On-coil Switched Mode Amplifier for Parallel Transmission in MRI” describes an on-coil current-mode class D (“CMCD”) amplifier that may be used to produce MRI transmission coil excitations at desired RF frequencies. The on-coil CMCD amplifier is capable of performing within or proximate to the bore of the MRI magnet or within less than one wavelength of the amplifier from the transmit coil. Providing an on-coil amplifier allows digital control signals to be sent to the coil assembly, improving synchronization between the transmission coils while reducing interference, cross talk, physical space requirements associated with cables, and heating normally associated with parallel transmission MRI systems. The on-coil CMCD amplifier disclosed in the '595 patent is well adapted for use in MRI systems having a typical magnetic field strength between 1 and 5 Tesla.
Today there is intensive research effort in high-field MRI systems having a magnetic field strength of around 7 Tesla. MRI systems having a higher magnetic field strength typically benefit from a higher signal-to-noise ratio (SNR) and consequently higher spatial and temporal resolution. The Larmor frequency at higher magnetic field strengths is proportionally greater as well (e.g. around 300 MHz for a 7T system). While the higher magnetic field is beneficial in many ways, the higher magnetic field and higher attendant excitation frequencies present unique challenges for developing electronic circuits that can operate on or proximate to the transmit coil.